Some relatively recent developments in the construction of radial piston hydraulic pumps and motors have led to greatly improved efficiency throughout a wider speed range and to a higher maximum operating speed capability. One such development is the radial piston hydraulic pump-motor described and shown in U.S. Pat. No. 3,709,104 granted Jan. 9, 1973 entitled "RADIAL PISTON HYDRAULIC PUMP OR MOTOR WITH LOW LOSS REACTION LINKAGE." The pump-motor described and shown in that patent uses Scott-Russell linkages between each of the pistons and the reaction assembly, thereby eliminating side loads between the pistons and cylinders and reducing friction losses and wear problems. Another improvement, one which has contributed both to greater efficiency and to greater speed capability, is described in Tobias U.S. Pat. No. 3,345,916 entitled "HIGH EFFICIENCY HYDRAULIC APPARATUS" granted Oct. 10, 1967. That patent involves an improvement in the porting of the pintle shaft and cylinder block which considerably reduces or eliminates a pressure drop/along the flow path from the charge area of the pintle shaft to each cylinder on the charge part of each piston stroke and a pressure rise along the path from each cylinder to the discharge area of the pintle shaft on the discharge part of each piston stroke, thereby greatly reducing turbulence, noise and cavitation. Tobias U.S. Pat. No. 3,548,719 entitled "HIGH EFFICIENCY RADIAL PISTON PUMP OR MOTOR WITH IMPROVED FLOW PATTERN" issued Dec. 22, 1970, describes a further improvement in the porting.
The latter two of the three patents mentioned above describe and show, more particularly, ports in the cylinder block which are elongated in the axial direction, each of which has, relative to the cylinders, a substantially lesser maximum circumferential dimension and greater maximum axial dimension. Preferably the cross-sectional area of each port in the cylinder block is substantially equal to the cross-sectional area of the cylinder bore, and the cross-sectional areas of the transfer passages between the ports and the cylinders are of uniform cross-sectional area throughout their lengths. Accordingly, there is no change in cross-sectional area in the flow path of fluid to and from the cylinders, and therefore, no significant drop in the hydrostatic pressure of the hydraulic fluid delivered to the cylinder or increase in pressure in the fluid returned from the cylinders. Moreover, the maintenance of a substantially uniform cross-sectional area throughout the extent of the transfer passages between the ports and cylinders reduces turbulence, noise and cavitation.
In all radial piston hydraulic pump-motors, the hydraulic fluid delivered to and discharged from the cylinders flows in the axial direction through a supply passage in the pintle shaft into a charge area and must turn through an angle of 90.degree. in the charge area and transfer passage to enter the cylinder radially; the fluid then returns back through the transfer passage in the block to the discharge area of the pintle shaft and in so doing turns through another 90.degree. angle for return through the return passage in the axial direction. The two 90.degree. changes in flow direction that occur as a body or mass of fluid is first delivered to a cylinder and then returned from a cylinder during each cycle produces turbulence in the flow, and the turbulence, in turn, involves an energy loss as far as the torque output of the pump-motor is concerned and an energy exchange in the form of a loss of hydrostatic pressure and an increase in heat or thermal energy.